Process for the preparation of leather dust



PROCESS FOR THE PREPARATION OF LEATHER DUST Frederick Bruce Forward, McGaheysville, Va.

No Drawing. Application October 5, 1953, Serial No. 384,350

4 Claims. (Cl. 260-118) This invention relates generally to a process for the preparation of leather dust and has particular application to a process for preparing leather dust which is especially adapted for use in the preparation of leather coating compositions.

Both vegetable and chrome (fully) tanned leathers are known to remain unaffected by immersion for about five or ten minutes in boiling water. It has been known heretofore to grind up scrap leather, or to boil scrap leather in Water for a period of about 12 hours in order to prepare leather dust. It is also known to boil scrap leather for an extended period of time in either an acid or in an alkaline aqueous solution. However, the various leather dusts prepared by known prior art methods and used in coating compositions have serious disadvantages. They have a tendency to peel off, or granulate from the finished leather composition itself, or they tend to disintegrate and decompose, or they result in a finish which cannot be tooled or decorated to give the same efiect as tooled or decorated leather. Moreover, they lack the resiliency, body and life which characterizes real leather. The prior art leather dusts generally have been found to be of Q limited commercial use in the preparation of real leather surfaces and coatings because these leather dusts have not posssesed the proper combination of physical characteristics necessary to achieve a leather surface which has the appearance and feel of genuine leather such as is used to decorate table tops, desk tops, doors, and various articles of manufacture.

The commercial value of a universal leather dust which may be used with various bonding agents to achieve successfully real leather surfaces and coating on paper, cloth, wood, metal, plastics such as the various thermoplastic and thermosetting molding resins, etc., becomes apparent when it is realized that the cost of the composition leather coating is almost negligible as compared to that of a leather surface comprising a unitary hide or skin.

In addition, it has been found that the practice of the various prior art methods results in a final product leather dust which includes a considerable quantity of leather wool as opposed to leather dust. Leather wool comprises a fibrous, wooly mass as opposed to leather dust particles and the presence of leather wool in the coating composition produces an inferior finished leather coating which lacks the characteristics of real leather. Consequently, it is of considerable advantage to reduce the formation of leather wool in the preparation of leather dust.

nited StatesPatcnt O 2,808,398 Patented Oct. 1, 195? Accordingly, it is an object of this invention to provide a process for the making of leather dust from cured or tanned leather, including scrap leather, left-overs or waste from hide, boots, shoes, etc., which process will result in a leather dust which is especially adapted for use in a leather coating composition which closely simulates an integral finished leather surface of the unitary hide or skin type.

A further object is to provide a leather flour or leather dust which may be admixed with a liquid bonding agent capable of setting readily into a relatively thin film, which holds the leather particles in position and also adheres to the surface of the base material, especially synthetic thermosetting resins, to produce a leather coating or finish having the characteristics of integral leather hides and skins.

A further object is to provide a leather flour or leather dust which is adapted for a dusting application onto a liquid bonding agent surface which has been sprayed or otherwise applied onto a base material such as wood, or metal in order to achieve a composition leather finish having the characteristics of real leather.

Another object of the invention is to provide a process for making a leather dust which will have the necessary characteristics to produce a resilient, non-flaccid lively finish when applied to a base material with a suitable bonding agent, as by spraying.

Still another object is to provide a process for making a leather dust which is universally usable for a variety of coating operations, including the coating of metal, wood, cloth, molded and prefabricated plastics, etc., and yet will exhibit the characteristics of real leather.

These and other objects of the invention will be readily understood by reference to the following description.

In general, the process of the present invention is accomplished by initially soaking tanned or cured leather in water heated, as by steam, for a time and temperature falling within the line of satisfactory soaking conditions shown on the graph in Example 1, in order to hydrolyze the leather, such as scrap, to the proper characteristics. This hydrolysis results in a rubbery leather mass which is characterized by substantially the same life and elasticity exhibited by a freshly water-soaked tanned or cured hide. The term hydrolysis is intended to include any decomposition reaction, extraction, dissolving or leaching which is caused by the soaking. The exact nature and identification of What occurs during treatment is not clearly understood, but it is believed that it includes both an extraction of the fats and oils and a hydrolysis of the fatty acid esters to the corresponding free fatty acids. In any event, it is believed that the soaking, as defined, results in the removal of the required amount of animal fats and oils from the tanned or cured leather, although the invention is not to be limited by theory.

Secondly, the rubbery leather mass is dried until it becomes a hard, solid mass. Thirdly, the hard, solid leather mass is reduced to a particle size prior to drying, as by kiln drying, and this is conveniently accomplished by cutting the leather up into chunks and pulverizing it to a screen size of about /8 of an inch, as by means of a hammer mill. Thereafter, the leather particles are dried at about F. for about 14 to 15 hours until a moisture content of about ten percent, and preferably about eight percent, is obtained. Alternatively, the leather particles may be dried and cured for a correspondingly shorter period of time at a temperature approaching 175 F. Higher temperatures are unsatisfactory in that they result in a change in the structure of the leather particles which may be caused by partial carbonization or partial decomposition. The drying and curing time also may be increased or decreased depending on the particle size employed; that is, the larger the particle size the longer the drying time and conversely. Thereafter, the leather is fully reduced to a particle size of or of an inch. Small amounts of leather wool are occasionally formed and this leather wool is' removed by blowing or screening or other convenient separating operation, and the leather flour or leather dust is then ready for use in the leather finishing or coating operation.

EXAMPLE I An ordinary burlap sack containing leather scraps comprising shoe-sole scraps, shoe-last scraps and shoe-top scraps was immersed in awooden tub of water heated by steam at 190 F. for'two hours, after which time it was observed that the leather had been hydrolyzed to the extent that it had the characteristics approximating those of a slice of fresh harm; that is, it was rubbery, somewhat elastic and resilient. In addition, it was observed that the leather had not been charred in any way, which would result in the complete loss of its liveliness. In this connection, it is preferable that the leather be out of contact with any metal inasmuch as the presence of metal in the hydrolyzing operation has been found to alter the characteristics of the final leather dust to such an extent that it has been found generally unsatisfactory for the purpose intended.

The leather was then dried until it formed a hard, solid mass comprising a single, unitary chunk as compared to the many initial scraps used as the starting material.

After the hydrolyzed leather has been completely dried, it was cut up into chunks of about 2 inches in size for convenience in the subsequent drying and pulverizing operations. The leather chunks were run through a hammer mill until a screen size of about inch was obtained.

Thereupon the leather particles were dried in a kiln at about 125 F. for about 14 hours in order to achieve a moisture content of about eight percent and also to properly cure the leather particles.

Thereafter the leather particles were further pulverized to V8 inch screen size and thereafter the particles were still further reduced to a screen size of about to 3 of an inch.

The resulting leather dust was then screened to separate therefrom a small amount of leather wool which is generally formed during the preparation of the leather dust in the pulverizing operation. In this example, it was found that parts by weight of leather wool resulted from the preparation of 700 parts of leather dust. It is essential to remove the leather wool since the resulting leather dust will not serve its intended purpose if any appreciable amounts of leather wool are contained therein. In this connection, it has been found that pulverized leather which has not been properly hydrolyzed in accordance with the present invention, may contain as much as 50 percent by weight of the leather wool; whereas completely unhydrolyzed raw leather dust will contain as high at 90 percent leather wool and only 10 percent of the usable leather dust.

This process was repeated on 51 other samples of substantially identical leather, each soaked under different time and temperature conditions. The dust from each sample was applied to a board in-the manner set forth hereinafter. The quality of the thus produced boards 'evaluated as commercially acceptable or not acceptable in relation to each soaking condition is set forth in the graph below.

hror o Soaking temperature in *F.

75 bilt).)

Efiect of soaking time and temperature on quality of leather board TooDark 0 0 X X X T00 coarse 180 XXXXXXXXXX TooDark and T00 coarse 170 XXXXXXXXXX Soaking Time in hours 0 indicates commercially acceptable boards. X indicatescommereially unacceptable boards.

EXAMPLE II the leather was dried and cured for about 8 hours at Various batches of scrap leather from different sources require some minor modifications in the treatment in order .to achieve a consistent standard quality of finished leather dust. One manner of determining the modifications necessary is to note whether or not the leather particles tend to gum up and adhere to each other during the kiln drying operation. This generally denotes the presence of incompletely tanned leather or indicates that either the temperature or the treating time, or both, should be extended to. their upper limits.

The leatherdust obtained by the practice of the pres- .ent process is especially adapted for incorporation with a suitable bonding agent such as synthetic resin, glue and the like. It has been found to be of particular use with a synthetic glue of the type known commercially as Dupont Elvacet, although other bonding agents have also been used successfully in the preparation of a suitable leather surface. It has been found that the leather dust prepared in accordance with the present invention will not deteriorate, crumble, disintegrate, change color or otherwise alter its physical properties when combined with a bonding agent to form a suitable leather coating or finish.

. It has further been found that the leather dust made in 6 accordance with the present invention has excellent storageand aging properties and is not subject to physical or chemicalchanges caused by aging or sunlight, which features become commercially important, particularly where large scale production of coatings or finishes are involved.

lnpractice, one/coating composition and method of preparation and application which has been found satisfactory is as follows:

COMPOSITION 8 parts dry leather dust prepared as per Example 1. 4% Darwin No.'2 by weight of dry leather dust. (An 7 anionic surface active agent consisting of the sodium salt of polymerized alkyl-aryl sulfonic .acids (R. T. Vander- 16 parts Elvacet by weight. (An aqueous dispersion of polyvinyl acetate containing 55-57% solids and having a viscosity of 8-10 poises (Du Pont).) parts Water. 6 drops Octanol (octyl alcohol).

METHOD OF COMPOUNDIN G The surface active agent is added to the leather under mlxing and the water is added to the synthetic glue under stirring. The leather mixture is sifted slowly into the Water-glue mixture, followed by slow and thorough stirring. The octyl alcohol is then added, followed by thorough stirring. The resulting leather finish composition is next strained through a 30-40 mesh screen. The flexibility of the composition may be increased by the addition of about 1 to 5% Resoflex and/or 2 to 5% by Weight of turkey red oil as plasticizers. (Resoflex is an alkyd resin type plasticizer for polyvinyl acetate and nitrocellulose.) The solids content is 100% and the acid number is 32. The viscosity as measured on the Gardner scale is Z-3 While the color using the Gardner standards is 5-7 (Cambridge Industries).

The above leather finish composition was applied by spraying onto a plywood panel with a conventional spray apparatus, care being taken to continuously agitate the supply of coating composition in the spray tank during the spraying operation so as to maintain an evenly distributed suspension of leather particles in the composition.

I do not desire to be bound by any theory although I have observed that my discovery involving the use of the critical temperature range and critical time range of hydrolysis, preferably in the absence of metal contact, enables the preparation of a leather dust which has been modified to the desired degree and extent necessary. My observations indicate that some hydrolysis and curing to take place, and these observations appear to have some merit, since neither raw leather dust nor leather dusts which have been boiled for extended periods of time, or at high temperatures, or in the presence of metals, acids or alkalies appear to possess the requisite characteristics.

This application is a continuation-in-part of my copending application Serial Number 128,679, filed November 21, 1949, now abandoned, for Process for the Preparation of Leather Dust.

While the invention has been described by means of specific embodiments, it will be understood that it is intended to cover minor variations and departures therefrom as coming within the spirit and scope of the appended claims.

I claim:

1. A process of preparing leather dust which comprises soaking leather selected from the group consisting of tanned leather and cured leather in water at a temperature of about 190" F. for about two hours to form a rubbery mass devoid of the original leather shape, drying and curing the resulting leather mass at about 125 F. for

6 about fourteen hours to a moisture content of about eight eight percent to form hard, solid, brittle chunks, reducing the particle size of said resulting dried and cured leather to a flour and dust particle size, and separating therefrom substantially all the leather wool produced.

2. A process of preparing leather dust which comprises soaking leather selected from the group consisting of tanned leather and cured leather in water at a temperature of about 190 F. for about two hours to form a rubbery mass devoid of the original leather shape, drying and curing the resulting leather mass at a temperature range above room temperature and below 175 F. to form hard, solid, brittle chunks, reducing the particle size of said resulting dried and cured leather to a flour and dust particle size, and separating therefrom substantially all the leather wool produced.

3. A process of preparing leather dust which comprises soaking leather selected from the group consisting of tanned leather and cured leather in Water for a time and temperature falling Within the line of satisfactory soaking conditions shown on the graph in Example I to form a rubbery mass devoid of the original leather shape, drying and curing the resulting leather mass at about F. for about fourteen hours to a moisture content of about eight percent to form hard, solid, brittle chunks, reducing the particle size of said resulting dried and cured leather to a flour and dust particle size, and separating therefrom substantially all the leather wool produced.

4. A process of preparing leather dust which com prises soaking leather selected from the group consisting of tanned leather and cured leather in Water for a time and temperature falling within the line of satisfactory soaking conditions shown on the graph in Example I to form a rubbery mass devoid of the original leather shape, drying and curing the resulting leather mass at a temperature range above room temperature and below F. to form hard, solid, brittle chunks, reducing the particle size of said resulting dried and cured leather to a flour and dust particle size, and separating therefrom substantially all the leather wool produced.

References Cited in the file of this patent UNITED STATES PATENTS 27,343 Baschnagel Mar. 6, 1860 228,463 Hyatt et a1. June 8, 1880 967,215 Manns Aug. 16, 1910 1,973,074 Howes et a1. Sept. 11, 1934 2,148,904 Horowitz Feb. 28, 1939 FOREIGN PATENTS 1,295 Great Britain of 1857 OTHER REFERENCES Progress in Leather Science, 1920-1945, page 417. 

1. A PROCESS OF PREPARING LEATHER DUST WHICH COMPRISES SOAKING LEATHER SELECTED FROM THE GROUP CONSISTING OF TANNED LEATHER AND CURED LEATHER IN WATER AT A TEMPERATURE OF ABOUT 190*F. FOR ABOUT TWO HOURS TO FORM A RUBBERY MASS DEVOID OF THE ORIGINAL LEATHER SHAPE, DRYING AND CURING THE RESULTING LEATHER MASS AT ABOUT 125*F. FOR ABOUT FOURTEEN HOURS TO A MOISTURE CONTENT OF ABOUT EIGHT EIGHT PERCENT TO FORM HARD, BRITTLE CHUNKS, REDUCING THE PARTICLE SIZE OF SAID RESULTING DRIED AND CURED LEATHER TO A FLOUR AND DUST PARTICLE SIZE, AND SEPARATING THEREFROM SUBSTANTIALLY ALL THE LEATHER WOOL PRODUCED. 